A thruster as here understood is a steerable propulsion device arranged mainly beneath the hull of a marine vessel. The thruster is formed of a propeller unit (rotatable/steerable round a vertical axis) beneath the hull and of a substantially vertical housing. The propeller drive may be arranged mechanically, hydraulically or electrically. Though the present invention covers all three drive options, the following exemplary description of the thruster concentrates on the structures required by the mechanical drive. The electric and hydraulic drives have been only briefly discussed.
The exemplary thruster, when viewed from the standpoint of the mechanical drive has three main parts, i.e. the upper gearbox, the vertical shaft, and the lower gearbox. The upper gearbox includes the upper gear transmission that is formed of a substantially horizontal drive shaft terminating to a pinion wheel, which transmits power to a larger gearwheel mounted on a substantially vertical upper gearbox shaft. The vertical shaft is normally formed of three parts, i.e. the upper gearbox shaft, a floating intermediate shaft, and a pinion wheel shaft. The intermediate shaft may be coupled to the upper gearbox shaft and to the pinion wheel shaft with flexible or floating shaft couplings or the intermediate shaft may be replaced with a flexible or floating shaft coupling. The lower end of the vertical shaft, i.e. the pinion wheel shaft is provided with a pinion wheel that transmits the power to a gearwheel mounted on a substantially horizontal propeller drive shaft. Both the pinion wheel and the gearwheel are located within the lower gearbox. The lower gearbox is also called a pod. In both gearboxes the rotational speed of the shafts receiving the power is reduced.
If the thruster has an electric or hydraulic drive the upper gearbox of the mechanical drive may be replaced with the electric or hydraulic drive. The shaft of the electric or hydraulic drive motor is vertical and connected, preferably by means of a flexible or floating coupling, to the intermediate shaft or directly to the pinion wheel shaft. The electric or hydraulic drive motor may sometimes be provided with a shaft extending down to the pinion wheel to form its shaft, too.
Since the thruster discussed in this specification is a steerable one, the thruster has to be made rotatable round the vertical axis. This means that the upper gearbox has to be kept stationary, while the rest of the thruster components are steered. To fulfil this requirement the upper gearbox is fastened by means of an annular cover plate to the hull structure of the marine vessel. The cover plate has an opening for the vertical shaft, and it is provided with at least one steering motor the shaft of which extends substantially vertically through the cover plate. The lower end of the shaft of the steering motor is provided below the cover plate with a steering gear pinion that rotates a ring-shaped gearwheel arranged on an annular flange mounted on a vertical shaft housing forming the frame structure of the steerable/rotating thruster. The vertical haft housing surrounds the vertical shaft and extends downwardly such that the lower gearbox is fastened to the lower end the vertical shaft housing. The vertical shaft housing is formed of an upper part called as an upper vertical shaft housing, and a lower part called as a lower vertical shaft housing. The upper vertical shaft housing surrounds the floating intermediate shaft, and the lower vertical shaft housing the pinion wheel shaft. The lower face of the cover plate is provided with a ring-shaped support member, the radially outer surface of which faces the radially inner surface of the ring-shaped gearwheel. A bearing supporting the weight of the vertical shaft housing and the lower gearbox is arranged in connection with the ring-shaped support member and the ring-shaped gearwheel. The upper vertical shaft housing is surrounded by a so-called stembox the outer wall (converging conically in FIG. 1) of which is arranged in connection with the hull structures of the marine vessel. The lower end of the stembox outer wall is provided with bearings supporting the vertical shaft housing and with sealings for keeping the lubrication oil within the stembox.
Below the bearings and the sealings the upper vertical shaft housing terminates to a flange to which the lower vertical shaft housing is attached. The lower vertical shaft housing, so-called shank forms a cavity through which the pinion wheel shaft runs and where the upper bearings of the pinion wheel shaft are located. To the lower end of the lower vertical shaft housing is the lower gearbox fastened. The lower gearbox, i.e. the pod is provided with the lower bearings of the pinion wheel shaft, and the propeller drive shaft with its bearings.
The lubrication of the steerable thruster has been arranged this far by either arranging full oil bath in both the stembox, the shank and the lower gearbox or arranging splash lubrication in each lubricating position. However, practice has shown that splash lubrication especially in the stembox is challenging, as part of the points requiring lubrication are at the level of the top of the stembox, i.e. the steering bearing and the gearwheels involved in steering. Thus full bath lubrication in the stembox is the preferred alternative. Though full bath lubrication ensures the best lubrication the practice has shown that full bath lubrication in the lower gearbox wastes substantial amount of energy due to gearwheels churning oil. This problem is especially severe when the thruster is a so-called ice-pod used in arctic environment. The ice-pod construction means, when compared to traditional open water thrusters, a relatively small propeller and a high propeller shaft speed, which results in higher energy consumption in the churning of oil.